i have to consult with my peers first.. lol
dremel with a diamond bit to widen the hole is that possible?
Why would anyone what to spend $7 for a bit and maybe even buy a dremel, then spend 1/2 hour cutting a hole with the possibility of cracking the tank?
Stopping a glass shop, ask them for a piece of 3"x3" 1/8" thick glass scrap and picking up a tube of silicone is alot safer and quicker. unless you cut your fingers off while holding the glass, or in your case, rear end somebody on the way.
Easy solution fix easy problems, glass holes are not easy to make bigger.